Magnetron heater circuit



y 1956 L. A. STEVENSON, JR

MAGNETRON HEATER CIRCUIT Filed Dec. 2, 1952 ,4. C. F/L. VOUS INVENTO R LOUIS A. STEVENSONJR.

ATTORNEY United States Patent MAGNETRON HEATER CIRCUIT Louis A. Stevenson, Jn, Boston, Mass., assignor to Sylvania Electric Products Inc., a corporation of Massachusetts Application December 2, 1952, Serial No. 323,546

3 Claims. (Cl. 315-106) The present invention relates to electric circuits, and in particular, to the control of the heating energy that is applied to the filament of a vacuum tube. While the problem is general in some respects, the present invention is particularly applicable to magnetrons whose energization is of pulse form. For this reason, the illustrative disclosure is directed to magnetrons.

In the operation of magnetrons, the thermionic cathode is heated by a suitable internal filamentary heater energized by a variable supply, corresponding to the magnetron plate current. With higher levels of plate current, or electron flow from cathode to anode, a certain portion of the electrons emitted by the cathode returns and heats the cathode by bombardment. Consequently, there should he a reduction of heater energy from the alternating current supply. that is conventionally used for the heater. The decrease in heater energy required bears virtually a linear relationship to the increase in magnetron plate cur rent.

An object of the present invention is to provide a new and effective circuit for automatically controlling the energy supplied to the heater of a magnetron or the like so as to vary it in inverse relationship to its plate current. A further object is to devise a circuit employing commonly available general-purpose components for controlling heater energization in a magnetron in inverse relation to the plate current.

The illlustrative embodiment is seen to employ a magnetron with the heater connected to a filament or heatersupply transformer as is conventional, and in this circuit there is interposed the primary winding of a conventional transformer whose secondary winding is variably loaded from open circuit to short circuit advantageously by means of paired thyratrons. The nature of the invention and further detailed advantages and features thereof will be appreciated from the following disclosures of an illustrative embodiment shown in the following drawings:

In the drawings,

Figure l is the wiring diagram of an illustrative embodiment of the invention.

Figure 2 is a graph demonstrating the effectiveness of the heater voltage control circuit in Figure 1.

in Figure l, magnetron 10, which is illustrated diagrammically, includes a cathode a, an anode 10b, and a heater 10c for heating cathode 10a to the proper temperature for desired thermionic emission. A pulse transformer 12 is connected between cathode 10a and anode tub for supplying high level bursts of energy or pulses to the magnetron in the conventional manner and at a standard repetition rate. The pulse transformer includes a bifilar secondary winding 12a whose terminals are at a common potential, insofar as the pulse energy is concerned. A heater transformer 14, having a primary winding 1411, connected to a source of constant alternating-current voltage, has its secondary winding 14b connected to heater 10c through secondary 12a of transformer 12. A transformer 16 has its primary winding 16a interposed between filament Winding 14b and the magnetron filamentary heat- 2,748,316 Patented May 29, 1956 er 100. The secondary winding 16b of this transformer is center-tapped and connected to the anodes of a pair of thyratrons 18, so as to energize the anodes thereof in alternation. Thyratrons 18 are provided with a suitable grid-energizing network including a negative direct-current bias supply 20, an alternating current supply 22, and a pair of resistance-capacitance phase shifters 24, one for each thyratron 18, so that the control effected by the thyratron grids will be sensitive and in the optimum relation to the phase of the alternating current of the thyratron anodes. Transformer 22 in an actual embodiment was simply a 6.3 volt heater transformer which was also utilized to energize the filamentary heaters of the thyratrons shown, a pair of miniature thyratrons in this actual embodiment. The thyratrons pass variable amounts of current in dependence on the times during the alternating current cycles that they become conductive, so that, the sooner they become conductive the larger will he the average current that they pass.

The magnetron anode current reaches the thyratron control network through a low-pass filter 26, including resistor 26:: and condenser 26b, and significantly, includes resistor 23. Condensers 30, connected to the side of pulse trans former 12 remote from the magnetron, are for the purpose of passing the magnetron anode-to-cathode pulse current, the resulting charge on these condensers then passing through filter 26 and resistor 28, between pulses, as a sustained direct current of virtually constant level through resistor 28. This resistor is seen to be interposed between the cathodes of thyratrons 18 and ground (where the magnetron anode is connected) the D.-C. magnetron current then acting to introduce a bias voltage between the thyratron cathodes and grids. This resistor does not carry the thyratron current which, as shown in the drawing, returns from the thyratron cathodes directly to the center tap of transformer secondary winding 16b. As the magnetron anode or plate current rises, the thyratron cathodes are thus driven progressively more positive relative to the thyratron grids. This reduces the current passed by the thyratrons and acts to increase the voltage drop across the primary winding that is interposed between the magnetron heater and its alternating current supply.

Heater ltlc may, for example, require 14 volts for proper heating during standby periods when no pulse power is being supplied at which time the cathode temperature is to be correct for immediate magnetron operation. As shown in Figure 2, the dotted line A represents the relationship between the amount of filament voltage that should be supplied to the magnetron in a typical case as the plate current of the magnetron rises. This plate current is, of course, the pulse current averaged by condensers 3t? and filter 26. In Figure 2, the optimum heater voltage required at 30 milliamperes of plate current is approximately 6 volts.

In the illustrative circuit of Figure l, transformer 14 is designed to supply 3.5 amperes at 21 volts at its secondary, so that transformer 16 is to introduce an alternating current drop in voltage of 7 volts during standby, rising to 15 volts at a magnetron plate current of 30 milliamperes. The variable bias on the thyratrons resulting from the current hrough resistor 28 is seen in Figure 2 to produce an actual net heater voltage available at filament as represented by the solid curve B. Thus, through this circuit the heater voltage is always adjusted almost ideally.

Whereas some control circuits devised in an effort to achieve the purpose of the circuit described above have used complex networks depending either on specialized and complicated saturable reactors, or elaborately controlled radio frequency power supplies for the filamentary heater, the foregoing circuit is seen to use readily available components of simple and compact nature and of relatively low-power rating.

a, V a 3 In the illustrated circuit, the thyratrons are shown as having screen grids connected together, and returned through a long time constant biasing circuit 32 shown as a parallel-connected resistor 'andcapacitor unit. This detail, as well as-other-details as ofgrid phasing and steadystate biasing, are naturallysusceptible to a variety of modifications without in any way affecting the underlying concept; Further modifications in detail, as well as other applications of the'inv'ention, will occur to those skilled in the art. Accordingly, the appended claims should be accorded such broad interpretation as is consistent with the spirit and scope of the invention.

What is claimed is:

, 1.- A magnetron heater supply circuit, including a magnetron having an anode and a cathode and an electrondischarge space between said cathode and said anode, and a heater for producing thermionic emission of the cathride in the magnetron, arr alternating-current supply having connections to said heater, a transformer having the primary Winding interposed in said connections so as to be connected inseries with the magnetron heater, said transformer having a secondary winding, a thyratron circuit connected as a controlled load on said secondary winding, a pulse power supply having energizing connections to said anode and said cathode including a condenser in series with the said anode-cathode electron-discharge space, and a biasing circuit for said thyratron circuit including a low pass filter and having input connections to said condenser so as to vary the degree of short-circuiting of said secondary winding as a function of the magnetron current.

2. A magnetron circuit having a controlled heater supply, including a magnetron having an anode or plate, a cathode and a heater, a pulse power supply connected to said plate and said cathode for producing high levels of pulse current, a condenser interposed between said cathode and'said pulse supply for-passingthe pulse current and a resistor in a circuit connected in parallel with said condenser and effective to develop a control voltage proportional to the average magnetron plate current, a constant voltage alternating-current supply having connections to said heater and a transfer having its primary winding interposed in said connections, said transformer having a center-tapped secondary winding, a pair of thyratrons havin the cathodes thereof connected to the center-tap of transformer secondary winding and having the anodes thereof connected to the terminals of said secondary winding, the control grids of the thyratrons having a biasing circuit, a return connection to the thyratron cathodes, said resistor being interposed in said return connection thereby to control the thyratrons as a variable current path across said secondary winding.

3. A magnetron circuit in accordance with claim 2 wherein said resistor is connected to said thyratron cathodes properly to drive the cathodes progressively more positive in proportion to the magnetron plate current, thereby to decrease the secondary winding current in said transformer and to increase the impedance of the primary winding thereof as a direct function of the magnetron plate current.

References Cited in the file of this patent UNITED STATES PATENTS 2,236,195 McKesson Mar. 15, 1941 2,319,378 Weisglass May 18, 1943 2,438,396 Jacobs et al Mar. 23, 1948 2,516,089 King July 18, 1950 2,519,173 Buder et al Aug. 13, 1950 2,626,366 Wright Jan. 20, 1953 4-1 l"f-4 AArAAA A 

